555 Timers

The 8-pin 555 timer is one of the most popular Integrated Circuits (ICs) ever made.
It is also one of the cheapest chips on the market, costing anywhere from $0.20 to $1.20 depending on the quantity and distributor.

The 555 timer will operate from a power supply of 4 V (some versions even less) to about 16 V.
The 14-pin 556 timer contains two 555 timers in a single package -- the two timers (A and B) share the same power supply pins.

Most 555 timers are connected to a couple of resistors and capacitors in one of these 4 configurations:

Astable - producing a square wave

Monostable - producing a single pulse when triggered

Bistable - a simple memory which can be set and reset

Buffer - an inverting buffer (Schmitt trigger)

For more details on variety of circuits that can be built using a 555 timer, see

Example

555 timer is an analog integrated circuit based on voltage comparison. If on pin 2 there is a voltage which is lower than 1/3 power supply, the output (pin 3) takes the high level. The circuit keeps that condition until the voltage on pin 6 goes over 2/3 power supply.
Pin 7 is an electronic switch, and we use it on delay applications, to discharge the capacitor. When the output takes the low level, pin 7 closes to the ground.
In the following diagram you can see a possible application, based on 555.
This simple circuit can be utilized to drive a monostable relay, using a single button switch.
When I press the button IC output (pin3) assumes the high level, but Q1 transistor swithces
off. So when I release it the relay is energized (now the exchange switch is closed on 5 and 9).

When I press the button again IC output assumes the low level, but Q3 transistor switches

on: when I release the button the circuit goes to the start condition.